The world around us is constantly changing, more so now than ever before in recorded history. Humans are dramatically affecting the natural world.

Between global warming and the exploding growth of the human population, which results in the need to convert more and more wild land to agriculture and housing, it is becoming increasingly difficult for species in the wild world to survive and thrive. Species have always gone extinct, and new ones are slowly evolving by genetic changes too subtle for humans to witness in their lifetimes. But now extinction is happening at a rate that is 1,000 times the normal background rate.

In fact, scientists say that we are now in the sixth great extinction. There have been five great extinctions of species in the past, the fifth one being when a comet crashed into earth 65 million years ago, resulting in the demise of dinosaurs.

The current great extinction began about 12,000 years ago, when the climate warmed abruptly in the Northern hemisphere and ended the last ice age. Yes, climate changes have happened in the past, and no, humans weren't responsible for them. But we are responsible for this one.

Over a period of 150 years, we have burned up a huge amount of the carbon that was sequestered in the form of petroleum, natural gas and coal. This carbon went into the atmosphere in the form of carbon dioxide. The physical chemistry of this gas is known to produce a greenhouse effect. Present-day earth warming was predicted in the late 1800s based on the knowledge of how carbon dioxide absorbs heat energy.

Population growth and the entry of large numbers of people into previously scarcely populated areas has also had an effect. Early settlers to this country wiped out the passenger pigeon, which had numbered in the billions. The Carolina parakeet in the Eastern and Midwestern U.S. was also hunted to extinction.

But amid this doom and gloom, there are bright spots of hope. While some people are oblivious to the destruction of the wild world, others are aware and care. Several species that were hunted to near extinction have been brought back from the brink. Bison and wolves in the lower 48 states are two notable examples.

I have just returned from a trip to Yellowstone National Park. Unfortunately Vic's class schedule didn't allow him to accompany me. Lacking my live-in biologist spouse, I hired Ken Sinay, a biologist and guide from Yellowstone Safari Co. (http://www.yellowstonesafari.com) to increase my chances of seeing wolves in the park. I was not disappointed.

Ken knew right where to go to find wild wolves, progeny of the ones released into the park in 1995. Wolves had been extirpated in the lower 48 states, deliberately killed off by hunters and ranchers. The last wolf in Yellowstone was killed in 1926.

Wolves were absent from the park for about 70 years. And their absence affected the ecosystem. Without predators other than grizzly bears, elk populations burgeoned to ecosystem-damaging levels.

Wolves are a natural component of the ecosystem. Since their re-introduction into Yellowstone, elk numbers have returned to more sustainable levels, and riparian vegetation along the rivers and streams has rebounded. The Yellowstone ecosystem is in better balance now.

The original pack of wolves has reproduced and thrived; there are now many packs in and around Yellowstone. A new pack of wolves is forming in Yellowstone now. Until they produce pups that survive, the pack won't be named. For now, it is known simply as the New pack.

My goal for the trip was met when I witnessed three wolves from this pack get up from their morning nap and trot off to an elk kill that they had made a couple of days earlier. It is such a thrill to see wolves in the wild that I kept my eyes glued to the spotting scope, not even thinking about snapping a photo.

One of the things that struck me the most about Yellowstone on this trip was the large number of bison that I saw. I have been visiting this national park since my first trip with my parents in 1958. Back then, the bison herd was managed to keep the herd at about 300 head. Sightings of bison were few and far between.

Now the herd has grown to about 5,000 head. I saw more bison on this trip than I had ever seen before.

The story of bison is one of tragedy and then success. In the 1800s, great herds migrated across the Great Plains. The number of bison at that time was estimated at 60 million. But buffalo hunters shot them willy nilly, often for just their tongues and hides. By the late 1800s, they were thought to be extinct.

But a small herd was found in Yellowstone country and protected from hunting. Their numbers have rebounded, and the species was saved.

On my trip to Yellowstone, bison sightings in the park were plentiful. At one point, I was standing on an overlook, looking down the slope at a herd of about 15 bison resting and chewing their cuds. Another herd approached from over a ridge to the left, moving steadily in near single file. That herd just kept coming and coming until about 200 of them were in sight. They were on the move, bulls bellowing and calves kicking up their heels. I got goosebumps seeing so many bison on the move. I felt like I had glimpsed the past.

*

A final perspective

Times constantly change, both in the natural world and in our work-a-day world. One of those unexpected changes has happened to us. Vic and I are sad to announce that after 12 years, our columns in the Independent are coming to an end. We have two more to write, and then you will just have to follow our activities on my blog at http://www.greenlifeinsocal.com.

VIC LEIPZIG and LOU MURRAY are Huntington Beach residents and environmentalists. They can be reached at Lmurrayphd@aol.com.

Facebook comments are not regularly monitored by Huntington Beach Independent staff. If you believe a comment violates any of the below rules, please email Editor John Canalis at john.canalis@latimes.com and Web Editor Ann Haley at ann.haley@latimes.com. Staff will review and remove offending comments during working hours, 8 a.m. to 6 p.m. Monday through Friday.

The Independent reserves the right to remove, block or refuse any comment for any reason. When posting to HBIndependent.com, you agree to not provide comments that:

* are defamatory, threatening, disparaging, grossly inflammatory, false, misleading, fraudulent, inaccurate, unfair, contains gross exaggeration or unsubstantiated claims, violate the privacy rights of any third party, are unreasonably harmful or offensive to any individual or community.

* discriminate on the grounds of race, religion, national origin, gender, age, marital status, sexual orientation or disability, or refer to such matters in any manner prohibited by law.

* "flame" any individual or entity (e.g., sends repeated messages related to another user and/or make derogatory or offensive comments about another individual), or repeat prior posting of the same message under multiple threads or subjects.

* violate or encourage the violation of any municipal, state, federal or international law, rule, regulation or ordinance.

* advertise, promote or offer to trade any goods or services, except in areas specifically designated for such purpose.

*contain copyrighted or other proprietary material of any kind without the express permission of the owner of that material.

* violate any right of hbindependent.com or any third party.

* upload or transmit viruses or other harmful, disruptive or destructive files, material or code.